A Swedish think tank says world stockpiles of nuclear warheads have fallen from 22,600 to 15,850 between 2010 and 2015, but nuclear armed states have continued to modernize their arsenals.

In a report published on June 15, the Stockholm International Peace Research Institute (SIPRI) said the United States and Russia represented the bulk of the reduction, but pointed to "extensive and expensive long-term modernization programs" in the two countries, which account for 90 percent of the nuclear weapons.

The report said the other three nuclear armed states legally recognized by the Nuclear Non-Proliferation Treaty -- China, France, and Britain -- are "either developing or deploying new nuclear weapon systems or have announced their intention to do so."

China was the only state among the five global nuclear powers to have a "modest" increase in the size of its arsenal.

Among the remaining nuclear nations, India and Pakistan continued to increase their arsenals, while Israel tested long-range ballistic missiles.